- Pygidium
The pygidium is the posterior body part or shield of
crustacean s and somearthropod s, such asinsect s and the extincttrilobite s. It contains theanus and, in females, theovipositor . It is composed of fused body segments, sometimes with a tail, and separated from thoracic segments by an articulation.In
arachnid s, the pygidium is formed by reduction of the last three opisthosomal segments to rings where there is no distinction between tergites and sternites. A pygidium is present inPalpigradi ,Amblypygi ,Thelyphonida ,Schizomida ,Ricinulei and in the extinct orderTrigonotarbida . It is also present in early fossil representatives ofhorseshoe crabs ."Pygidium" is also a superseded
genus offish of the familyTrichomycteridae . Most species of this genus have been reassigned to the genus "Trichomycterus ".Trilobite
In trilobites, the pygidium can range from extremely small (much smaller than the head, or
cephalon ) to larger than the cephalon. They can be smooth, as in orderAsaphida , or spiny, as in orderLichida .References
Shultz, J.W. (1990). Evolutionary Morphology And Phylogeny of Arachnida. Cladistics 6: 1-38.
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